In rats under urethane, multibarrelled microelectrodes were used to record field responses of CA1 hippocampal pyramids evoked by fimbrial-commissural stimulation. A strong potentiation of population spikes could be obtained by either a local release of ACh or by a brief tetanic stimulation of the medial septum (which by itself evoked only minimal field responses of the hippocampus). Both effects were sharply reduced after intravenous injections of large doses of atropine or scopolamine, indicating that the septo-hippocampal innervation is to an important degree cholinergic and muscarinic. However, since dimethylphenylpiperazinium (a nicotinic ganglionic agonist) can also evoke population spikes, a significant though variable component of this cholinergic action may operate via nicotinic receptors.